In many operations conducted in petroleum wells, an operator employs different tools or other articles which do not move readily through the well bore. This problem of movement is compounded in deviated holes, where the weight of a tool string combined with the angle of the well bore contributes to the problem. In some instances, the string becomes stuck in the well bore and further operations are impossible until the string is freed. Jarring tools, or "jars," are commonly employed in strings to help free a string should it become stuck.
Hydraulic jars, such as are disclosed in U.S. Pat. Nos. 3,399,740 and 3,429,389, issued to Burchus Q. Barrington and assigned to the assignee of the present application, have been employed for some time. In general, such jars employ a mandrel within an outer case, there being a hydraulic fluid in several communicating reservoirs between the two. When a pulling force is applied to the mandrel, hydraulic fluid moves between reservoirs in a highly impeded manner, thus inhibiting mandrel movement. When the mandrel travel reaches a certain point, the impedance is bypassed, resulting in a sudden, forceful movement of the mandrel with respect to the case. A hammer element on the mandrel then impacts on an anvil element in the case, producing a substantial jarring force in the string. Repeated reciprocation of the mandrel with respect to the case is generally sufficient to free the string in the well bore. These prior art jars, however, possess a number of disadvantages. They are incapable of numerous repetitions without replacement of parts and reassembly termed "redressing." Furthermore, the jars may be affected adversely by well bore fluids infiltrating the hydraulic fluid. Moreover, the force of impact obtained with these jars is inconsistent over a number of repetitions. Additionally, no slidable elastomeric seal is employed between the mandrel and outer case, a desirable feature which allows greater pressure buildup prior to bypassing, but which cannot be employed successfully due to the structuring of the bypass area, which would promote seal destruction. Finally, the disclosed jars cannot be redressed in the field, but must be taken to a shop facility.
U.S. Pat. No. 4,196,782, issued Apr. 8, 1980 and assigned to Dresser Industries, Inc. discloses another hydraulic jar of the type discussed above, which employs a vortex jet metering element to initially impede the flow of hydraulic fluid. While such a vortex jet element provides somewhat more consistency of fluid flow, the manner in which the jet is mounted in the assembly leaves much to be desired, as there is no screening assembly to prevent particulate matter in the hydraulic fluid from clogging the jet and the jet appears to be mounted with adhesive, which can clog the jet during assembly of the tool. Furthermore, the bypass for the hydraulic fluid is merely an enlarged bore in the case, again preventing the use of a sliding elastomeric seal between the mandrel and case due to deterioration caused by the force of the bypassing hydraulic fluid and return action of the mandrel. An interference fit to provide the mandrel-case seal is called for, but it is readily apparent that such a fit would deteriorate due to wear after several reciprocations of the jar, thus allowing leakage past the metering jet and preventing the necessary high pressure buildup prior to bypassing, which pressure buildup results in the required large force during the subsequent bypassing movement of the mandrel.
U.S. Pat. Nos. 4,023,630, and 4,200,158 each disclose hydraulic jars of a relatively complex structure seeking precision performance, but at the expense of longterm reliability and repeatability due to the large number of individual elements and seals employed. Moreover, the complexity of these jars prohibits easy field maintenance and reassembly.